feat: expose provider capability bounds to app server clients (#20049)

follow up of #19442. The app server now exposes provider-derived bounds
through a new v2 `modelProvider/read` method. The response reports the
configured provider map key as `modelProvider` and returns the effective
capability booleans so clients can align their UI with the same
provider-owned limits used by core.

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -26,6 +26,7 @@ mod mcp_server_status;
 mod mcp_tool;
 mod memory_reset;
 mod model_list;
+mod model_provider_capabilities_read;
 mod output_schema;
 mod plan_item;
 mod plugin_install;